The present invention relates to a sampling device for thermal analysis of solidifying metal, especially for thermal analysis in the production of castings.
Thermal analysis is a technique for monitoring variations in temperature change of molten substances during solidification to be able to determine the microstructure and hence properties of the substances in solid form. This is accomplished by taking a sample from the melt, transferring it into a sampling vessel and recording and evaluating a time-dependent temperature change in the sample during solidification, by means of temperature responsive sensor means, such as thermocouples or other devices known in the art.
When using thermal analysis for recording the solidification of molten metals, such as compacted graphite iron (CGI), it is important that the analysis is done under the same geometric and thermal conditions, as will occur in the castings. The contribution of the sampling vessel is to control the cooling during solidification of the sample of the molten metal so that the geometric and thermal conditions, in the sampling vessel are similar to those obtained in the castings. The ability to accurately measure the solidification behaviour of the molten metal allows a foundry to control its process and guarantee high quality in production.
Sampling vessels for thermal analysis are known in a large number of designs. They can be made of graphite, for use in aluminum melts, or made of a ceramic material, when intended for use with molten cast iron. However, they cannot be made of steel due to dissolution and/or thermal imbalance.
A drawback for many vessels is that they are made of materials, which are difficult to machine. Another drawback is that since they are immersed in the bulk of the metal, when taking a sample, the risk of thermal shock cracking is a problem, especially when they are made of ceramic materials, whereby they easily crack.
WO-A1-96/23206 (incorporated by reference) describes a sampling vessel to be immersed in a bath of molten metal to be analyzed. A sampling vessel is disclosed, a double-walled steel vessel with a space between the walls, having low radial thermal conductivity. The space can be filled with an insulating gas, such as air. The inner wall of the vessel is thin and thus provides a low heat capacity, so as it will easily obey a steady-state thermal condition in a short time. Furthermore, heat lost from the outer surface of the inner wall, is not let out into the ambient atmosphere, because of an outer wall, acting as a radiation shield, surrounding the inner wall and the insulating space between the walls.
The previously mentioned sampling vessel in WO-A1-96/23206 is very well suited for thermal analysis for use in CGI production on account of its special properties. However, it is expensive to manufacture, which is a disadvantage from the point of view that it can only be used once. The thermocouples are at proper locations, i.e., one near the inner wall and one at a position, which attempts to simulate the centre of a hypothetical sphere of molten metal with uniform heat-loss per unit area. In fact the heat-loss from the bottom part is much lower, as compared to the top. One reason for this non-uniform behaviour is that the open top part emits much more heat per unit area than the rounded bottom part. Another reason is that the contact between the two surfaces at the upper joint allows heat to go around the insulting air space. This is a considerable disadvantage, since proper results are not always obtained.
It is of great importance to cool at a similar rate as the castings which are to be controlled. Equilibrium cooling would take too long to be of any practical value for this process control situation, for instance CGI production, since results would not be available before the casting process was completed, nor would it form a similar material micro-structure.
Furthermore it is essential that the sampling device is not expensive, since it can only be used once. Since especially accurate measuring elements, such as thermocouples are expensive, it is preferred to reuse them several times. A major drawback with many known sampling devices is that the rather expensive thermocouples are only used once.
Yet another drawback is that it is difficult to produce large series of sampling vessels, at low cost wherein all vessels show similar properties regarding geometric and thermal conditions etc.
The object of the invention is to overcome these considerable disadvantages by using an improved sampling device, with a sampling container having controlled hear-loss per unit area, which simulates a sphere of molten metal, since a sphere is the most uniform, and therefore most reliable and accurate shape for thermal analysis. This sampling device having controlled heat-loss per unit area simulates a spherical solidification of the molten metal inside the sampling container, but is not spherical in shape, because of manufacturing limitations for instance. The sampling device according to the invention comprises a double-walled container, provided with a radiation shield at the top and controlled space between the walls, which has much more controlled heat-loss, does not fail at high temperatures, is not expensive and has unproved positioning of the temperature responsive sensor means, such as thermocouples, which can easily be removed and reused.
Another important problem that is solved by the invention is the shifting of a thermal centre of the simulated sphere of molten metal, which shifts downwards once the exposed top surface of the sample inside the container solidifies.
All these requirements are achieved by providing the container having the features disclosed in appended claim 1. Up till now no such sampling device has been available.
The sampling device is intended for single use, is cheap, gives reproducible geometric and thermal conditions.
It has been found that heat issuing from the exterior surface of the inner wall of the container must not immediately be let out into the ambient atmosphere, as this would make it very difficult to accomplish a controlled, slow and reproducible heat removal rate. Thus, the purpose of the outer wall is to define, together with the inner wall, a space between the walls that controls where hear is lost from the bottom and sides of the solidifying metal.
Thus, the space between the inner and the outer wall is an important parameter in regulating the heat loss due to radiation and thermal conduction. When this space is evacuated, or filled with a transparent material, such as air, radiation will be an important heat transfer mechanism. As temperature of the solidifying metal in the sampling device increases, radiation will be of increasing importance, since it""s effect increases with the fourth power of absolute temperature.
By selecting and fully or partly filling the space with a suitable medium, and/or by altering the thickness of the space, it is possible to adapt the geometry of the heat removal rate of the sampling device to the values required by thermal analysis. The medium may bc any known and suitable medium, such as, sand, vermiculite, mica, magnesia, chlorite, various ceramics or combinations thereof, but is preferably a gas, such as air, because of cost. In one preferred embodiment, a distance (d1) between the walls in the flattened bottom part of the container is only 5-50%, preferably about 20% of a distance (d2) between the side-walls of the container, thereby increasing the heat loss due to conduction from the bottom. Because of the reduced space at the bottom of the container, heat-loss due to conduction is increased at the bottom, balancing heat-loss from the open top of the container thereby simulating heat-loss as occured by spherical solidification of a sphere of molten metal.
Another parameter of greatest importance is the shape of the container. To be able to position temperature responsive sensor means for thermal analysis, enclosed in a protective tube, at a certain distance from the inside surface of the inner wall, the container has a flattened bottom surface. Because of practical reasons during manufacturing, both walls, i.e. the inner wall and the outer wall are made with flattened bottoms. The area of the flattened bottom part of the inner wall is at least as large as the area of a protective tube comprising sensor means to allow for a constant distance to the end of said tube. In one preferred embodiment, the diameter of said flattened bottom part area is twice the diameter of the area of the protective tube, preferably larger. The protective tube, which is partly immersed in the solidifying metal in the container has tide end to the bottom surface closed. An open protective tube does not work, since sensor means will be destroyed easily.
Furthermore, the open top part (mouth) of the container is of most importance. Heat-loss will normally be large, if not covered by a lid. Thus, the support member (See FIG. 1, item 15), comprising a radiation shield, which positions the thermocouples and is attached by legs to the container, acts as a lid to reduce radiation heat-loss from the top of the metal sample. Otherwise, the top part of the sampling device would act as a cold body, absorbing too much radiation from the hot container. This simulates heat-loss from a sphere of molten metal under solidification, as it balances slower heat-loss from the bottom of the container.
The amount of heat liberated by the initial formation of flake graphite in the near-wall region is very small, and indeed insufficient to be relied upon as a control parameter. However, if the shape of the bottom sample of the container is predominantly spherical; and, if the sampling device is preheated (for example by immersion into molten iron) thus avoiding formation of a chill zone of solidified iron in the near-wall region: and, if the sampling device is allowed to hang freely, so that heat is not extracted into a floor or mounting stand, a favourable convection current will develop within the molten iron contained in the sampling device. These convection currents xe2x80x9crinsexe2x80x9d the flake graphite away from the pre-heated upper walls of the container in the sampling device and effectively concentrate the flake growth in a flow-separated region at the base of the essentially spherical container.
The inner wall of the container is preferably made rather thin and/or of a material with a low specific thermal capacity, in order to impart a desirable low total thermal capacity to the inner wall. In addition to this the inner wall has preferably a high total heat transfer coefficient, to equalize the temperature of the sample quantity and the wall; and as the total heat transfer coefficient is high, the time required to transfer the amount of heat will be short.
The inner wall can be made of any material that has the thermal properties stated as above, being thermally stable in the molten metal being sampled. Typically a metal or alloy is used. Materials that are inexpensive and ensure reliable series production, especially steel, are preferred.
It also possible to alter the colour and/or the surface finish to modify the radiation characteristics of the walls.
The interior surface of the inner wall of the container is preferably coated with a protective barrier in order to protect said wall from dissolving into or react with the hot liquid metal sample. Such protective coating could also be applied to the exterior surface of the outer wall, particularly if the container is intended to be immersed in hot liquid metal when sampling. The protective coating does not affect the thermal balance, since it is very thin. However, the coating is critical in defining the solidification behaviour. The coating can be inert or doped with reactive substances to consume Mg and induce flake graphite formation near the bottom temperature responsive sensor means. This is disclosed in Swedish patent application 9704208-9 (incorporated by reference) and for further information Swedish patent application 9003289-7 (incorporated by reference). The protective coating could be any refractory oxide such as alumina, magnesia, zirconia, silica carbide, etc.
The temperature responsive sensor means are mounted in a support member, guiding and crimping the sensor means in place. The sensors for thermal analysis are generally thermocouples, although the present invention is not limited in that sense; any kind of sensor suited for thermal analysis of solidifying metal can be used. C. f. Swedish parent application 9600720-8 infrared pyrometry (incorporated by reference).
The sensor means for thermal analysis, in the following thermocouples, are enclosed in a concentrically arranged protective tube, which is partly immersed in the solidifying metal in the container. The protective tube having one closed end is positioned and held by the support member and a thereto arranged radiation shield, i.e. firmly at two points. One thermocouple (gauge part) is inserted to the closed end of said tube, in a way that it can easily be removed. It is also critical to locate a second thermocouple measurement point (gauge part) within the thermal centre to avoid that the cooling curves shift during the measurement period. Therefore, a second thermocouple (gauge part), also removable, is inserted into the protective tube, at the thermal centre of the molten sample, preferably at a distance, c, about ⅔ of the total height a, of the inner container. Because the thermocouples are removable, they can be reused for a number of measurements. The tube enclosing the two thermocouples, is positioned as close as possible to the flattened inside bottom surface in the container, but must not contact this surface. It is essential to assure that the protective tube is completely surrounded by solidifying metal where no voids or bubbles could interfere with the measurement. Furthermore, it is of great importance that the thermosensors are firmly mounted, so that they do not move sideways, dug analysis. Inaccurate positioning of the thermocouples, is a considerable disadvantage, which interferes heavily with the measurement results. This is avoided as the thermocouples are accurately positioned by the support member and thereto arranged radiation shield, crimping the protective tube, comprising the sensors, accurately in position. The distance of the closed protective tube to die flattened bottom, b, is 1-10% of the total height of the inner container, a, preferably about 5%.
The protective tube can be made of materials, such as steel, preferably stainless steel or quartz glass. Steel tubes normally require coating. The invention is not limited to the use of only one protective tube or one pair of thermocouples. As many as necessary can be used at different distances, i.e. measurement points.